Process for the separation of diene from organic mixtures

ABSTRACT

Dienes are separated from organic mixtures comprising diene and alkene having one double bond by contacting the mixture against one side of a cyanoethyl cellulose membrane and withdrawing at the other side a vaporous mixture having increased diene concentration. Exemplary of the organic mixtures is a mixture of butadiene and butene. The cyanoethyl cellulose perferably has a degree of substitution (D.S.) of from about 0.57 to 3.0.

United States Patent Perry et al.

[ July 17, 1973 [75] Inventors: Eli Perry, St. Louis; William F.

Strazik, St. Ann, both of Mo.

[73] Assignee: Monsanto Company, St. Louis, Mo.

[22] Filed: Mar. 8, 1972 [21] Appl. No.: 232,976

[52] US. Cl. 55/16 [51] Int. Cl 1.; B0ld 53/22 [58] Field of Search55/16, 158

[56] References Cited UNITED STATES PATENTS 2,953,502 9/1960 Binning etal 210/23 Binning et al 55/16 Cahn 55/16 Primary ExaminerCharles HartAtt0rneyLynden N. Goodwin et al.

[57] ABSTRACT Dienes are separated from organic mixtures comprisingdiene and alkene having one double bond by contacting the mixtureagainst one side of a cyanoethyl cellulose membrane and withdrawing atthe other side a vaporous mixture having increased diene concentration.Exemplary of the organic mixtures is a mixture of butadiene and butene.The cyanoethyl cellulose perferably has a degree of substitution (D.S.)of from about 0.57 to 3.0.

6 Claims, No Drawings PROCESS FOR THE SEPARATION OF DIENE FROM ORGANICMIXTURES BACKGROUND OF THE INVENTION 1. Field of the Invention Thisinvention relates to a process for separating dienes from organicmixtures containing same. In a particular aspect this invention relatesto a process for the separation of diene from organic mixturescomprising diene and alkene having one double bond by preferentialpermeation through a membrane under pervaporation conditions. In a moreparticular aspect this invention relates to a process for the separationof diene from an organic mixture comprising diene and alkene having onedouble bond by contacting said mixture against one side of a cyanoethylcellulose membrane and recovering on the other side a vaporous mixturerich in diene.

2. Description of the Prior Art Processes for the preparation of dienessuch as butadiene and isoprene yield reaction mixtures which containorganic reaction products (typically substituted and unsubstitutedC;,--C hydrocarbons) in addition to organic solvents-and the desireddiene. Separation of dienes from such organic reaction media has beenaccomplished by distillation procedures. Principally because of theclose boiling points of dienes-and typical reaction byproducts,especially the corresponding alkenes having one double bond high refluxratios or azeotropic agents and costly distillation equipment arerequired for the distillation separation procedure.

Separation of components of azeotropie mixtures of organic materials bypervaporation through polymer membranes is known to the art from U.S.Pat. No. 2,953,502 issuedSept. 20, 1960 to R. C. Binning and Robert J.Lee. Among the membranes indicated as being useful in such separationsare membranes comprised of cellulose and of certain cellulosederivatives.

SUMMARY OF THE INVENTION It has been discovered in accordance with thepresent invention that dienes are effectively separated from organicmixtures comprising diene and alkene having one double bond bycontacting the mixture against one side of a cyanoethyl cellulosemembrane and withdrawing at the second side a vaporous mixture having ahigher concentration of diene than the aforesaid mixture. Cyanoethylcellulose membranes employed in the process of the present invention arehighly efficient in separating diene from other components of suchorganic mixtures using pervaporation separation techniques. The presentinvention is further advantageous in that it per mits avoidance ofcostly distillation procedures.

DETAILED DESCRIPTION The process of the present invention comprisescontacting an organic feed mixture comprising diene and alkene havingone double bond against one side of a membrane comprising cyanoethylcellulose membrane and withdrawing at the second side a mixture having ahigher concentration of the preferentially permeable diene than theaforesaid feed mixture. It is essential that the mixture at the secondside he maintained at a lower chemical potential than that on the feedside. It is also essential that the product be withdrawn at the secondside in the vapor state. In the commercial utilization of the process,multi-stage operation is feasible in permeate (CA/CH) in permeant whereC, and C are the concentrations of the preferentially permeablecomponent and any other component of the mixture or of the sum of othercomponents respectively.

In carrying the process of the present invention, the first or feed sideof the membrane is such that the activities of the components aregreater than the activities on the second side. Preferably the firstside is above atmospheric pressure and the second side below atmosphericpressure. Still more preferably, the second side is maintained such thatthe pressure differential is greater than 0.01 atmosphere or preferablyfrom about 0.1 to about 0.5 atmosphere. A further preferred mode ofoperation is with the second side maintained at a vacuum of greater than0.2 mm. Hg.

The term Chemical Potential is employed herein as described by Olaf A.Hougen and K. M. Watson (Chemical Process Principles, Part II, JohnWiley, New York, 1947). It is related to the escaping tendency of asubstance from any particular phase. For an ideal vapor or gas, thisescaping tendency is equal to the partial pressure so that it variesgreatly with changes in the total pressure. For a liquid, the change inescaping tendency as a function of total pressure is small. The escapingtendency always depends upon the temperature and concentration. In thepresent invention, the feed substance is typically a liquid solution andthe other side of the membrane is maintained such that a vapor phaseexists. A vapor feed may be employed when the mixture to be separated isavailable in that form from an industrial process or when heat economiesare to be effected in a multi-stage process.

The feed side may be at pressures less than atmospheric, but preferablygreater than atmospheric and also at pressures over and above the vaporpressure of the liquid components. The collection or permeate vapor sideof the membrane is preferably less than atmospheric pressure, but underproper feed side conditions, also may be greater than atmosphericpressure. The total pressure on the feed side is preferably be tween 0psi absolute and 5,000 psig.

The conditions are always such as to maintain a higher chemicalpotential on the feed side than on the collection or vapor side.

The temperatures on the feed side and the collection side may vary overa wide range. However, temperatures should be avoided which causesubstantial decomposition of any of the organic materials in the mixtureor of the membrane and which cause the vapor pressure on the collectionside of any of the permeated materials to be exceeded by the pressurebeing maintained on that side. Typically, an increase in temperaturecauses an increase in permeation rate. A dramatic increase in rate oftenoccurs when the temperature exceeds the glass transition temperature ofthe polymer membrane being used in the separation procedure.

The process of the present invention provides for the separation ofdiene from organic mixtures comprising diene and alkene having onedouble bond. Such dienes are substituted and unsubstituted and typicallycontain from four to eight carbon atoms. A diene may be substitutedwith, for example alkyl, aromatic and halogen substituents. Typicalorganic components and mixtures from which the dienes are separateinclude C C alkenes such as butene, hexene, propylene, and heptene aswell as other hydrocarbons such as chlorohexane, acrylic acid, octane,propane, etc. and the like. Separations are carried out by removal ofthe preferentially permeable diene through the membrane with the saiddiene, in a higher concentration than in the feed, being recovered fromthe collection side of the membrane as a vapor and with the impositionof a state of lower chemical potential on such collection side of themembrane. For example, a mixture of butadiene and butene may be appliedto one side of a flat diaphram or membrane to accomplish removal of atleast a part of the butadiene, leaving a more highly concentrated butenesolution at the feed side of the membrane or diaphram. A state of lowerchemical potential is maintained on the collection or downstream side ofthe membrane by vacuum, e.g., from 0.1 mm to the vapor pressure of theorganic component of the mixture which has the lowest vapor pressure atthe membrane at the respective temperature as long as the vapor phase ispresent on the downstream side.

In the system referred to above, the butadiene selectively passesthrough the membrane with the butadiene-rich composition being rapidlyremoved as vapor from the collection side of the membrane.

In contrast to the present invention, the employment of permeates inliquid phase on the second side of the membrane is impractical becausethe applied pressure has been found to be prohibitively high, e.g., upto 1,000 atmospheres being necessary because of osmotic pressures.Liquid-liquid permeation is largely an equilibrium phenomenon unless theosmotic forces are overcome while in contrast liquid-vapor orvapor-vapor permeations are rate controlled processes even at moderateconditions, in which the vapor is removed as soon as it reaches thecollection surface of the membrane. Liquid-vapor and vapor-vaporseparations are accordingly much more effectively carried out thanliquidliquid separations.

The permeation membrane used in the process of the present inventioncomprises cyanoethyl cellulose. The membrane may be a simple disc orsheet of the membrane substance which is suitably mounted in a duct orpipe, or mounted in a plate and frame filter press.

Other forms of membranes may also be employed such as hollow tubes andfibers through which or around which the feed is supplied or isrecirculated with the product being removed at the other side of thetubes as a vapor. Various other shapes and sizes are readily adaptableto commercial installations. The membrane, of course, must be insolublein the organic separation medium. Membrane insolubility" as used hereinis taken to include that the membrane material is not substantiallysolution-swellable or sufiiciently weakened by its presence in thesolution to impart rubbery" characteristics which can cause creep andrupture under the conditions of use, including high pressures.

The cyanoethyl cellulose preferably has a degree of substitution (D.S.)of in the range of from about 0.5 to about 3.0 with a degree ofsubstitution of in the range of from about 1.5 to 3.0 being especiallypreferred. The molecular weight of the cyanoethyl cellulose may varyover a wide range, but in all cases should be sufficient to permit thepolymer to be formed into a film which is sufficiently strong towithstand separation processing conditions. Cyanoethyl cellulose havinga molecular weight in the range of from about 50,000 to about 5,000,000is typically employed.

The membranes may be prepared by any suitable procedure such as, forexample, by casting a film or spinning a hollow fiber from a dope"containing polymer and solvent. Such preparations are well known to theart.

An important control over the separation capacity of a particularmembrane is exercised by the method used to form and solidify themembrane (e.g., casting from a melt into controlled atmospheres or fromsolution into baths at various concentrations and temperatures).

The art of membrane usage is well known with substantial literaturebeing available on membrane support, fluid flow and the like. Thepresent invention is practiced with such conventional procedures andapparatus. The membrane must, of course, be sufficiently thin to permitpermeation as desired, but sufficiently thick so as to not rupture underthe pressure conditions employed. Typcally suitable membranes have athickness of from about 15 to about 10 mils.

The following examples illustrate specific embodiments of the presentinvention. In the examples the membranes employed were in the form offilm disks and were mounted in a membrane holder. All membranes wereprepared by casting from solution.

EXAMPLE I A. Membrane permeations were conducted for the purpose ofseparating 1,3-butadiene from an organic liquid consisting of 80 weightpercent 1,3-butadiene and 20 weight percent trans2-butene usingcyanoethyl cellulose membranes. Each membrane was approximately 1 mil inthickness. In each run, preferential permeation of l,3-butadiene waseffected. The results are shown in the table. In each run, the pressureon the liquid side was higher than the pressure on the vapor side andwas that pressure caused by the vapor pressure of the mixture at thetemperature of separation.

B. For comparative purposes the above procedure was repeated in allessential details with the exception that the membranes employed werecellulose and cellulose triacetate. The results are shown in the table.

The results in the table show that cyanoethyl cellulose membranes aresuperior to cellulose membrane and cellulose triacetate membranes inseparating butadiene from butene under pervaporation separationconditions.

TABLE RateXlO g/hr/l 1.3 cmlmil Sepa- Run No. Membrane of membraneration Temp. thickness Factor (C) l cyanoethyl cellulose (2.5) 3 3.7 0 23.3 20 3 3.1 40 4 I40 2.5 60 5 cyanoethyl cellulose (2.7) l3 3.l 0 6 362.9 20 7 52 3.0 20 8 cyanoethyl cellulose (3.0) 37 3.0 20

9 cyanoethyl cellulose (l.85)* l l 3.4 20 i cellulose 0.3 3.5 llcellulose triacetate 1400 2.5 20

Degree of substitution EXAMPLE 2 The procedure of Example 1 is followedto separate isoprene from a liquid mixture of isoprene, hexene, andpentane using a membrane which comprises cyanoethyl cellulose having adegree of substitution of 2.5.

While the invention has been described with reference to particularembodiments thereof, it will be appreciated that modifications andvariations are possible without departing from the invention.

We claim:

1. A process for the separation of diene from an organic mixturecomprising diene and alkene having one double bond which comprisescontacting the said mixture against one side of a membrane comprisingcyanoethyl cellulose and withdrawing at the second side a vaporousmixture having a higher concentration of diene than the aforesaid feedmixture with the mixture at the second side being maintained at a lowerchemical potential than at the feed side.

2. The process of claim 1 wherein the pressure on the second side of themembrane is less than atmospheric pressure and lower than the pressureon the other side of the membrane.

3. The process of claim 1 wherein the feed mixture is a liquid mixture.

4. The process of claim 1 wherein the feed mixture comprises butadieneand butene.

5. The process of claim 1 wherein the membrane comprises cyanoethylcellulose having a degree of substitution of in the range of from about0.5 to 3.0.

6. The process of claim 4 wherein the cyanoethyl cellulose has a degreeof substitution in the range of from about 1.5 to 3.0.

2. The process of claim 1 wherein the pressure on the second side of themembrane is less than atmospheric pressure and lower than the pressureon the other side of the membrane.
 3. The process of claim 1 whereiN thefeed mixture is a liquid mixture.
 4. The process of claim 1 wherein thefeed mixture comprises butadiene and butene.
 5. The process of claim 1wherein the membrane comprises cyanoethyl cellulose having a degree ofsubstitution of in the range of from about 0.5 to 3.0.
 6. The process ofclaim 4 wherein the cyanoethyl cellulose has a degree of substitution inthe range of from about 1.5 to 3.0.